This happened to my 2012 XC60 RD T6, I changed all six and within 4 months 4 more coils burn up/melted. It ended up being a bad/faulty thermostat. Change the thermostat and no more issues!
Owner reviews for thermostat
Replaced the new Mahle/Behr one with a new Wahler one, and now it runs at 97° under low load and 80° under high load, as it should, and seems to be fine.
Only ever use a genuine Subaru thermostat.
I replaced the thermostat myself. It’s in a pretty bad place, hidden under the intake manifold. It took me about 3 hours to get the job done, had to use a mirror to see the screws. Mine was stuck open so the car in winter time wouldn’t even reach 190 F. I bought an OEM Wahler 07K121113B thermostat from Amazon for like 30$, so far has been working great.
Make sure you use a Mazda OEM thermostat. I wouldn’t trust anything but OEM for those.
I will say I went through several OEM stats and finally put a duralast in and it's been in now 5 years no problems.
Parts 03F-010-501-B Sensor 04E-121-113-G
Can confirm. I bought a stant thermostat because it's the only place I could find one. They sent me a used motorad with no gasket in the wrong temp that was stuffed in a stant box.
I had two Wahlers and they leaked as soon as I fitted them. In exactly the way you described.
I got the wahler one in 2019, that would explain the really slow leak I have.
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